Polymeric marker with high radiopacity

ABSTRACT

High radiopacity is achieved in a polymeric marker by combining a polymeric resin, a powdered radiopaque agent having uniformly shaped particles of a specific particle size distribution and a vetting agent. The method to produce the marker calls for the blending and pelletization of these materials followed by extrusion onto support beading. The resulting supported tubing is subsequently cut to length with the beading still in place. After ejection of the beading remnant the marker is slipped into place on the device to be marked and attached by melt bonding. Marking of a guidewire allows lesions to be measured while the marking of balloon catheters allow the balloon to be properly positioned relative to a lesion.

BACKGROUND OF THE INVENTION

The present invention is directed to elongated intracorporeal devices, and more particularly intraluminal devices for stent deployment, percutaneous transluminal coronary angioplasty (PTCA), and the similar procedures. PTCA is a widely used procedure for the treatment of coronary heart disease. In this procedure, a balloon dilatation catheter is advanced into the patient's coronary artery and the balloon on the catheter is inflated within the stenotic region of the patient's artery to open up the arterial passageway and increase the blood flow through the artery. To facilitate the advancement of the dilatation catheter into the patient's coronary artery, a guiding catheter having a preshaped distal tip is first percutaneously introduced into the cardiovascular system of a patient by the Seldinger technique through the brachial or femoral arteries. The catheter is advanced therein until the preshaped distal tip of the guiding catheter is disposed within the aorta adjacent the ostium of the desired coronary artery. A balloon dilatation catheter may then be advanced through the guiding catheter into the patient's coronary artery until the balloon on the catheter is disposed wthin the stenotic region of the patient's artery.

Once properly positioned across the stenosis, the balloon is inflated one or more times to a predetermined size with radiopaque liquid at relatively high pressures (e.g., generally 4-12 atmospheres) to dilate the stenosed region of a diseased artery. After the inflations, the balloon is finally deflated so that the dilatation catheter can be removed from the dilatated stenosis to resume blood flow.

Similarly, balloon catheters may be used to deploy endoprosthetic devices such as stents. Stents are generally cylindrical shaped intravascular devices that are placed within a damaged artery to hold it open. The device can be used to prevent restenosis and to maintain the patency of blood vessel immediately after intravascular treatments. Typically, a compressed or otherwise reduced diameter stent is disposed about an expandable member such as a balloon on the distal end of the catheter, and the catheter and stent thereon are advanced through the patient's vascular system. Inflation of the balloon expands the stent within the blood vessel. Subsequent deflation of the balloon allows the catheter to be withdrawn, leaving the expanded stent within the blood vessel.

Typically, the distal section of a balloon catheter or other percutaneous device will have one or more radiopaque markers in order for the operator of the device to ascertain its position and orientation under X-ray or fluoroscopy imaging. Generally, a band or ring of solid radiopaque metal is secured about an inner or outer shaft of a balloon catheter to serve as a radiopaque marker. Such configuration, however, locally stiffens the catheter shaft and thereby imparts an undesirable discontinuity thereto as the solid metal bands are relatively inflexible compared to a polymer balloon catheter shaft. Additionally, the metallic markers are relatively expensive to manufacture and relatively difficult to positively affix to an underlying device.

As is described in U.S. Pat. No. 6,540,721, which is incorporated herein by reference, many of the problems associated with the use of conventional markers may be overcome by replacing the rigid precious metal tubing with a polymer that is filled or doped with a suitable radiopaque agent. Such marker may be formed by blending a polymer resin with a powdered, radiographically dense material such as elemental tungsten and then extruding the composition to form a tubular structure with an appropriate inner diameter and wall thickness. The extrusion may then be cut to discrete lengths and installed onto the intended component via a melt bonding process.

A shortcoming of such an approach has been found to be the apparent limit to which a suitable polymer can be filled with a radiographically dense material to yield a composition that can be successfully compounded, economically shaped into suitable dimensions for markers and easily assembled onto a component without unduly compromising the desirable properties of the polymer matrix. The fill ratio that is achievable will determine how thick a marker must be in order to achieve a particular degree of radiopacity. In the case of tungsten in a polymer such as Pebax, the fill ratio limit has heretofore been found to be about 80 weight percent. Such weight percentage equates to about 18 volume percent which requires the marker to be excessively thick in order to achieve adequate radiopacity.

A polymeric marker is therefore needed having a substantially higher fill ratio than has heretofore been possible. Such marker would allow devices to be rendered highly visible without an inordinate increase in overall profile nor a compromise of the flexibility of the underlying component.

SUMMARY OF THE INVENTION

The present invention overcomes the shortcomings of previously described polymeric radiopaque markers by enabling a polymer to be filled or doped with a considerably greater quantity of a radiopaque agent than has heretofore been achievable. The increased fill ratio nonetheless allows uniform pellets to be compounded and an extrusion with the appropriate wall thickness to be formed. The resulting marker provides an unprecedented combination of radiopacity and flexibility. Such marker would allow any of various intraluminal devices to be radiopaquely marked including, but not limited to, coronary, peripheral, and guiding catheters as well as guidewires.

The marker of the present invention relies on the use of radiopaque materials with a preselected particle shape and a preselected particle size distribution as well as the inclusion of one or more additives in the polymer/radiopaque agent blend. A multifunctional polymeric additive is added to the composition in order to enhance the wetting, adhesive and flow properties of the individual radiopaque particles by the polymer so as to cause each particle to be encapsulated by the polymer and thereby allow the polymer to form a continuous binder. An antioxidant may optionally be added in order to preserve the high molecular weight of the polymer matrix as it is exposed to the high temperatures and shear stresses associated with the compounding and extrusion processes.

While previous attempts to increase fill ratios have involved tungsten powder of relatively fine particle size, the present invention relies on the use of particles of increased size in order to achieve such end. An increase in particle size has been found to allow the polymer to more effectively function as a continuous binder and thereby increase ductility at a given fill ratio or maintain ductility at increased fill ratios. It has been found that in constraining the average particle size to at least 2 microns and limiting maximum particle size to about 20 microns provides the desired results. In the case of tungsten in Pebax, a fill ratio of about 91.3 weight percent (equivalent to 36.4 volume percent) is readily attainable. Additionally, it has been found that the process by which the tungsten powder is produced has a considerable effect on both particle size distribution as well as the shape of the individual particles. Tungsten powder produced by either a “pusher” process or “atomization” process, then milled and classified has been found to provide discrete particles having a more equiaxed shape and size respectively and are therefore more ideally suited for the purposes of the present invention than powders produced employing a “rotary” process.

The marker of the present invention is manufactured by first tumble mixing the polymer resin with a pelletized wetting agent, such as maleic anhydride graft polyolefin resin (MA-g-PO), and an antioxidant and then introducing the mixture into the primary feeder of a twin screw extruder. The mixture is fed in at a controlled mass flow rate and conveyed down the barrel length as it is heated above its melting temperature and blended. At a point downstream, tungsten powder is introduced into the mix at a controlled mass flow rate via a secondary feeder. The tungsten powder and the molten ingredients become intimately intermixed as they are conveyed downstream and discharged through a die as molten strands which are cooled in water and subsequently pelletized. The markers are subsequently formed by extruding the tungsten filled polymer onto a continuous beading of PTFE and drawn down to yield the desired wall thickness. The extrusion is then cut to the desired lengths, preferably with the beading still in place so as to provide support. Removal of the beading remnant then allows the marker to be slipped onto the medical device or component thereof to be marked and melt bonded in place. Reliance on melt bonding obviates the need for the marker to completely surround the underlying device. Markers can for example be longitudinally split in half to form two markers of C-shaped cross-section. Or, solid strands of extruded marker material may be melt bonded to one side to form one or more longitudinal stripes or helical patterns.

Due to its high radiopacity, flexibility and melt bondability, the marker of the present invention is readily attached to for example the inner member of a balloon catheter, a guidewire, and even a guide catheter tip. The attachment of radiopaque markers of known dimensions to a guidewire or the attachment to a guidewire of multiple radiopaque markers with known separation distances impart a measurement capability to the catheter that allows a physician to quickly and easily measure lesions and decide on appropriate stent lengths.

These and other features of the present invention will become apparent from the following detailed description of preferred embodiments which, taken in conjunction with the accompanying drawings, illustrate by way of example the principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged side view of the radiopaque markers of the present invention attached to a balloon catheter;

FIG. 2 is an enlarged side view of radiopaque markers of the present invention attached to a guidewire in a preferred configuration; and

FIG. 3 is an enlarged side view of the radiopaque markers of the present invention attached to a guidewire in an alternatively preferred configuration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a radiopaque marker for use on a variety of devices that is flexible, highly radiopaque and is easily attachable to such devices by melt bonding. These properties allow markers to be of minimal thickness and thereby minimize the effect the marker has on the overall profile and stiffness of the device to which it is to be attached.

In order to achieve the high fill ratios that are necessary to attain the desired radiopacity and in order to do so without compromising the compoundability and workability of the polymeric material nor its ultimate strength and flexibility, a number of different parameters have been found to be of importance. More specifically, both the particle shape and particle size of the radiopaque agent must be carefully controlled while the inclusion of a MA-g-PO in the polymer blend is critical. An antioxidant may additionally be included in an effort to reduce the adverse effect the high processing temperatures and shear stresses may have on polymer properties.

A number of polymeric materials are well suited for use in the manufacture of the markers of the present invention. The material preferably comprises a low durometer polymer in order to render the marker sufficiently flexible so as not to impair the flexibility of the underlying medical device component to which the finished marker is to be attached. Additionally, the polymer must be compatible with the material of which the component is constructed so as to allow the marker to be melt bonded in place. The polymer must also impart sufficient strength and ductility to the marker compound so as to facilitate its extrusion and forming into a marker, its subsequent handling and attachment to a medical device and preservation of the marker's integrity as the medical device is flexed and manipulated during use. Examples of such polymers include but are not limited to polyamide copolymers like Pebax, polyetherurethanes like Pellethane, polyester copolymers like Hytrel, olefin derived copolymers, natural and synthetic rubbers like silicone and Santoprene, thermoplastic elastomers like Kraton and specialty polymers like EVA and ionomers, etc. as well as alloys thereof. The preferred polymer for use in the manufacture of a marker in accordance with the present invention is polyether block polyamide copolymer. A durometer of 25 or lower is preferred.

A number of different metals are well known to be radiographically dense and can be used in a pure or alloyed form to mark medical devices so as to render them visible under fluoroscopic inspection. Commonly used metals include but are not limited to platinum, gold, iridium, palladium, rhenium and rhodium. Less expensive radiopaque agents include tungsten, tantalum, silver and tin, of which tungsten is most preferred for use in the markers of the present invention.

The control of particle size has been found to be of critical importance for achieving the desired ultra high fill ratios While efforts to increase fill ratios have previously utilized small average particle sizes (1 micron or less) so as to minimize the ratio of particle size to as-extruded wall thickness, it has been found that higher fill percentages can be realized with the use of somewhat larger average particles sizes. It is desirable in the formulation of high fill ratio compounds to have the following attribute: 1) uniform distribution of the filler particles, and 2) continuity of the surrounding polymer matrix, and 3) sufficient spacing between filler particles so that the polymer matrix provides ductility to the bulk mixture to impart processability in both the solid and molten state.

The use of larger average particle sizes results in greater spacing between filler particles at a given percentage, thus maintaining processability during compounding and especially subsequent extrusion coating. The upper limit of average particle size is determined by the wall thickness of the coating and the degree of non-uniformity tolerable (ie., surface defects). It has been found that a particle size distribution having an average particle size range of at least 2 microns to 10 microns and a maximum particle size of about 20 microns yields the desired fill ratio and provides for a smooth surface in the marker made therefrom.

The control of particle shape has also been found to be of critical importance for achieving the desired ultra high fill ratios. Discrete particles of equiaxed shape have been found to be especially effective, as individual particles of irregular shape, including agglomerations of multiple particles, have been found to adversely impact the surface, and thus, the maximum fill ratio that is attainable.

It has also been found that the process by which certain metal powders are produced has a profound effect on the shape of the individual particles. In the case of metallic tungsten, the powders may be formed by the reduction of powdered oxides through either “rotary”, “pusher” or “atomization” processing. Of these processes, “rotary” processing has been found to yield the least desirable shape and size distribution as partial sintering causes coarse agglomerates to be formed which do not break up during compounding or extrusion and thus adversely effect the marker manufactured therefrom. Atomized powders have been reprocessed by melting and resolidifying “rotary” or “pusher” processed powders and result in generally equiaxed, discrete particles which are suitable for use in the present invention. “Pusher” processed powders are preferred due to their low cost and discrete, uniformly shaped particles.

In order for the polymer to most effectively encapsulate individual radiopaque particles, it is necessary for a low-energy interface to exist between such particles and the polymer so as to enable the polymer to “wet” the surface of the particles. Certain additives such as surfactants and coupling agents may serve as wetting agents and adhesion promoters for polymer/metal combinations that are not naturally compatible. It has been found that additives containing maleic anhydride graft to a polyolefin backbone provide a significant benefit in this regard wherein materials commercially available as Lotader 8200 (having LLOPE Backbone) and Licomont AR504 (having PP backbone) were found to be particularly effective for use with tungsten/Pebax combinations. Emerging extrusions were found to be less susceptible to breakage and the melt viscosity during compounding was lower as was manifested by a reduction in torque exerted during the extrusion process. The use of such additives allowed compounds with higher fill ratios to be successfully produced.

The inclusion of an antioxidant in the marker composition has also been found to be of benefit. A commercially available antioxidant such as Irganox B225 have been found to minimize degradation (i.e., reduction in molecular weight) of the polymer matrix as it is exposed to the multiple heat and shear histories associated with the compounding, extrusion, and bonding processes.

The compound used for the manufacture of the marker of the present invention is preferably made by first blending the polymer resin and wetting agent, and optionally, an antioxidant such as by tumble mixing after which such blend is introduced into a twin-screw extruder via a primary feeder. The feed rate is carefully controlled in terms of mass flow rate to ensure that a precise fill ratio is achieved upon subsequent combination with the radiopaque agent. The heat that the materials are subjected as they are conveyed through the extruder causes the polymer to melt to thereby facilitate thorough homogenization of all of the ingredients. The radiopaque agent powder, selected for its uniform particle shape and controlled particle size distribution as described above is subsequently introduced into the melt stream via a secondary feeder, again at a carefully controlled mass flow rate so as to achieve the target fill ratio. The solid powder, molten polymer and additives are homogenized as they are conveyed downstream and discharged through a die as molten strands which are cooled in water and subsequently pelletized. The preferred extrusion equipment employs two independent feeders as introduction of all components through a single primary feeder would require significantly higher machine torques and result in excessive screw and barrel wear. The powder feeder is preferentially operated in tandem with a sidefeeder device, which in turn conveys the powder through a sealed main barrel port directly into the melt stream. A preferred composition comprises a fill ratio of 91.3 weight percent of tungsten (H. C. Starck's Kulite HC600s, HC180s and KMP-103JP) to Pebax 40D. A maleic anhydride source in the form of Licomont AR504 is initially added to the polymer resin at the rate of approximately 3 pphr while an antioxidant in the form of Ciba Geigy B225 at the rate of approximately 2 pphr (parts per hundred relative to the resin). The temperature to which materials are subjected to in the extruder is about 221° C.

Once the marker material has been compounded, the marker can be fabricated in suitable dimensions by an extrusion coating process. While free extrusion is possible, this method is problematic due to the high fill ratios of the polymeric materials. Extrusion onto a continuous length of beading has been found to lend the necessary support for the molten extrudate to prevent breakage. The support beading may take the form of a disposable, round mandrel made of PTFE, teflon coated stainless steel wire or other heat resistant material that does not readily bond to the extrudate. By additionally limiting the area draw down ratio (ADDR) to below 10:1 the tungsten-laden melt can successfully be drawn to size by an extrusion puller. The beading provides the added benefit of fixing the inner diameter and improving overall dimensional stability of the final tungsten/polymer coating. Extrusions of the 91.3 weight percent fill ratio tungsten/Pebax composition described above over 0.0215″ diameter PTFE beading were successfully drawn down to a wall thickness of 0.0025″ to yield a marker properly sized for attachment to for example a 0.022″ diameter inner member of balloon catheter. Also, extrusion coatings of 91% compound over 0.007″ teflon coated stainless steel wire were successfully drawn down to single wall thicknesses of 0.002″ to make guidewire coatings.

Once the extrudate has cooled, the extrusion is simply cut to the desired lengths (e.g., 1 to 1.5 mm) of the individual markers, such as with the use of a razor blade and reticle, preferably with the beading still in place to provide support during cutting. The beading remnant is subsequently ejected and the marker is slipped onto a medical device or a particular component thereof. Finally, the marker is attached to the underlying substrate, preferably with the use of heat shrink tubing and a heat source (hot air, laser, etc.) wherein the heat (˜171-210° C.) simultaneously causes the marker to melt and the heat shrink tubing to exert a compressive force on the underlying molten material. Heat bonding a marker onto an underlying component provides the added benefit of slightly tapering the edges of the marker to reduce the likelihood of catching an edge and either damaging the marker or the medical device during assembly or handling of the medical device.

A marker formed as per the above described compounding, fabricating and assembling processes, having a fill ratio of 91.3 weight percent (36.4 volume percent) with a wall thickness of 0.0025″ has been shown to have dramatically more radiopacity than commercially available 80 weight percent compounds and comparable to the radiopacity of 0.00125″ thick conventional Platinum/10% Iridium markers.

FIG. 1 illustrates two radiopaque markers 12 attached to the inner member 14 of a balloon catheter 16. The markers attached to the inner member prior to the positioning of the inner member within the balloon 18 and attachment thereto at 20. Fluoroscopic illumination of the device allows the invisible balloon to be positioned relative to a lesion by virtue of the visibility of the radiopaque markers and their known positions relative to the balloon.

FIG. 2 illustrates a preferred embodiment of a guidewire with a measurement feature 22 wherein a series of radiopaque markers 24 are attached to the guidewire 24 at preselected separation distances 26 allow the device to be used as a type of ruler to measure the size of a lesion. The separation between adjacent markers may be controlled by the use of a radiotransparent tubular spacers 28 that are similarly melt bondable to the underlying guidewire. Upon assembly of the radiopaque markers and the radiotransparent spacers onto the guidewire, heat shrink tubing of sufficient length is slipped over the entire section of guidewire and heated to the appropriate temperature to cause both the markers as well as the spacers to become melt bonded to the guidewire.

FIG. 3 illustrates an alternatively preferred embodiment of a guidewire with a measurement feature 30 wherein an equally spaced series of differently sized radiopaque markers 32 a-e are attached to a guidewire 34 to allow the device to be used to gauge the size of a lesion. The separation between adjacent markers may be controlled by the use of a radiotransparent tubular spacers 36 that are similarly melt bondable to the underlying guidewire. Upon assembly of the radiopaque markers and the radiotransparent spacers onto the guidewire, heat shrink tubing of sufficient length is slipped over the entire section of guidewire and heated to the appropriate temperature to cause both the markers as well as the spacers to become melt bonded to the guidewire.

While a particular form of the invention has been described, it will be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the invention. More specifically, a variety of different polymers and radiopaque agents can be compounded using the appropriate wetting agent, markers of different shape and dimensions can be formed and the markers can be attached to any of a variety of medical devices that can benefit from being radiopaquely marked. Accordingly, it is not intended that the invention be limited except by the appended claims. 

1. A radiopaque marker, comprising: a) a polymer; b) radiopaque particles disposed within said polymer having an average diameter of at least 2 microns and a maximum diameter of about 20 microns; and c) a wetting agent for facilitating encapsulation of said particles by said polymer.
 2. The radiopaque marker of claim 1, wherein said radiopaque particles comprise greater than 18 volume percent of said marker.
 3. The radiopaque marker of claim 2, wherein said radiopaque particles comprise approximately 36 volume percent of said marker.
 4. The radiopaque marker of claim 1, wherein said polymer comprises Pebax and said radiopaque particles comprise tungsten powder.
 5. The radiopaque marker of claim 4, wherein said multi-functional polymeric additive comprises MA-g-PO.
 6. The radiopaque marker of claim 1, wherein said radiopaque particles are substantially equiaxed.
 7. The radiopaque marker of claim 6, wherein said particles are produced by a pusher process.
 8. The radiopaque marker of claim 6, wherein said particles are produced by an atomization process, resulting in a substantially spherical particle.
 9. The radiopaque marker of claim 1, further comprising an antioxidant.
 10. The radiopaque marker of claim 1, wherein said polymer is thermoplastic.
 11. The radiopaque marker of claim 1 formed so as to define a tubular structure.
 12. A method for manufacturing a radiopaque marker, comprising: a) providing a polymer, MA-g-PO and anti-oxidant (AO); b) causing said polymer to melt and to become intimately intermixed with said MA-g-PO; c) combining said molten polymer and wetting agent with radiopaque particles, wherein such particles comprise at least 30 volume percent of said combination and said particles have an average diameter of at least 2 microns and a maximum diameter of about 20 microns; d) extruding said combination onto support beading so as to form a coating thereon; and e) cutting said coating to preselected lengths.
 13. The method of claim 12, wherein said polymer is caused to melt and to become intimately combined with said wetting agent by conveyance through a compounding extruder.
 14. The method of claim 13, wherein said radiopaque particles are combined with said molten polymer, MA-g-PO, and AO in said compounding extruder.
 15. The method of claim 12, wherein said combination is extruded such that it is drawn while supported by said beading.
 16. The method of claim 12, wherein said combination is pelletized before being extruded onto said support beading.
 17. The method of claim 12, wherein said polymer comprises Pebax, said radiopaque particles comprise tungsten, said multi-functional polymeric additive comprises MA-g-PO, and said anti-oxidant comprises Irganox B225.
 18. A method of radiopaquely marking a medical device, comprising: a) providing a flexible polymeric radiopaque marker containing greater than 30 volume percent radiopaque particles, wherein such particles have an average diameter of at least 2 microns and a maximum diameter of about 20 microns; b) accurately positioning said marker on said medical device; and c) melt bonding said marker in place.
 19. The method of claim 18, further comprising disposing a length of heat shrink tubing about said accurately positioned marker and heating said tubing.
 20. The method of claim 18, wherein said radiopaque particles comprise tungsten.
 21. The method of claim 18, wherein said medical device comprises a balloon catheter and said marker is positioned on an inner member disposed within an expandable balloon.
 22. The method of claim 18, wherein said medical device comprises a guidewire.
 23. The method of claim 22, wherein a plurality of said markers are bonded to said guidewire.
 24. The method of claim 23, wherein said markers are equally spaced relative to one another so as to function as ruler under fluoroscopic inspection.
 25. The method of claim 23, wherein said guidewire includes a radiopaque feature and wherein said markers are each spaced a preselected distance from said feature.
 26. The method of claim 23, wherein said markers each have a different length so as to function as gauges under fluoroscopic inspection.
 27. A device for measuring lesion sizes, comprising: a) a guidewire; and b) at least one flexible polymeric radiopaque marker is melt bonded to a distal end of said guidewire, wherein said marker contains radiopaque particles comprising at least 30 percent of its volume.
 28. The device of claim 27, wherein two or more of said markers are melt bonded to said guidewire at preselected spacing relative to one another so as to function as a ruler under fluoroscopic inspection.
 29. The device of claim 27, wherein said guidewire includes a radiopaque feature and said marker is melt bonded a preselected distance therefrom.
 30. The device of claim 27, wherein said marker has a preselected length so as to function as a gauge under fluoroscopic inspection.
 31. The device of claim 27, wherein a plurality of said markers are bonded to said guidewire, each of a different length so as to function as a gauges under fluoroscopic inspection.
 32. The device of claim 27, wherein said radiopaque particles comprise tungsten.
 33. The device of claim 32, wherein a wetting agent facilitates encapsulation of said radiopaque particles by Pebax.
 34. The device of claim 33, wherein said wetting agent comprises maleic anhydride.
 35. A radiopaque, elastomeric composition, comprising: a) a polymer; b) radiopaque particles disposed within said polymer having an average diameter of at least 2 microns and a maximum diameter of about 20 microns; and c) a wetting agent for facilitating encapsulation of said particles by said polymer.
 36. The composition of claim 35, wherein said radiopaque particles comprise greater than 18 volume percent of said marker.
 37. The composition of claim 36, wherein said radiopaque particles comprise approximately 36 volume percent of said marker.
 38. The composition of claim 35, wherein said polymer comprises Pebax and said radiopaque particles comprise tungsten powder.
 39. The composition of claim 38, wherein said multi-functional polymeric additive comprises MA-g-PO.
 40. The composition of claim 35, wherein said radiopaque particles are substantially equiaxed.
 41. The composition of claim 40, wherein said particles are produced by a pusher process.
 42. The composition of claim 41, wherein said particles are produced by an atomization process, resulting in a substantially spherical particle.
 43. The composition of claim 35, further comprising an antioxidant.
 44. The composition of claim 35, wherein said polymer is thermoplastic.
 45. The composition of claim 35 formed so as to define a tubular structure. 